For the exploration of oil and gas, wells are drilled, which connect the oil/gas reservoir to the surface. The well is drilled by a cutting tool such as a drill bit attached at the bottom of the drill string that is rotated by a rig at the surface. The drill string may include a plurality of pipe (i.e., the drill pipe) coupled end to end to be thousands of meters long. The lower part of the drill string is called the Bottom Hole Assembly (BHA) and consists of specialty tools and heavier thick-walled pipes, such as drill collars and mud motors. With the drill bit attached to the BHA, the drill bit is on the bottom of the wellbore, and the upper end of drill string is held by the rig. As such, the drill pipe portion of the drill string is therefore constantly in tension while the BHA is partly in compression. Furthermore, fluids are introduced into the wellbore by being pumped through the drill string and out through nozzles of the drill bit. From the drill bit, the fluids return to the surface via an annulus between the drill string and wellbore to transport cuttings from the bit to the surface and lubricate the drilling process.
During subsurface drilling by the drilling rig, the drill string may experience various forces and torques as it rotates within a wellbore. The drill string may be rotated by a top drive or a kelly, to make a drill bit turn at the bottom of the wellbore. Torque is applied to sustain the rotation, as the drill bit may be in contact with the bottom hole to perform drilling, and friction may be present between the drill string and the wellbore. Due to the presence of torque along the drill string, the drill string may be twisted with a deformation angle that increases with depth, storing elastic energy with the deformed structure. In addition, the inertia affects the rotation of the drill string during periods in which its speed of rotation changes. Furthermore, lateral displacement may occur during rotation due some centrifugal effect. As radial movements are limited by the presence of the well-bore, radial shocks occurs and the rotating string bounces back with more risk of future radial shock. When such process is sustained, it may develop into sustained whirling pattern. Such successive repeated shocks may be forwards whirling of backwards whirling, with nutation of the center of rotation of the rotary element.
With each contact with the wall, a tangential impulse is generated due to the rotation of the tubular. This tangent force applied on the external surface of the tubular is obviously the origin of a torque impulse. These frictional torque impulses may result into short reduction of rotational speed of the drill string. Whirling is a high frequency process (several contacts and impulses per turn) of radial shocks involving radial displacement. As such, the frequency of contact may be 3 to 20 times per turn. Such a frequency may be 5 to 60 hertz depending on the drill-string RPM. The drill string acts as a low pass filter by the combination of the rotational inertia and torsional rigidity. The excited high frequency effect does not transmit over long distance; however, the consumed torque at each impact must be provided by the surface drive system and appears as an increase of the average torque.
Furthermore, “stick-and-slip” may occur along the drill string and especially at the drill bit and BHA (including stabilizer). Stick-and-slip refers to irregular rotational movement of a drill string due to the forces and torques caused by variation of the friction at the drill bit (cutters) and drill string elements (e.g., a hole bottom, a liner, a casing, a wall of the wellbore, cuttings, etc.) and the wellbore. Such unsteady friction at the contact points causes the drill string to slow down and possibly stops (stick). When considering “stick-and-slip” at the bit face of PCD bit, this effect may be generated by the cutters which may vary their depth of cut (penetration into the well-bore bottom face) due to unsteady WOB. For example, teeth of a drill bit can lock in a hole-bottom due to a sudden increase of axial load (weight-on-bit or “WOB”). The required rotary torque immediately adjusts to this effect. When considering “stick and slip” along the BHA (collar surface or stabilizer blades), the effect may be generated by the dependence of friction factor on the relative velocity of the elements versus the bore wall. Typically, the magnitude of the friction factor increases as the rotational velocity of the drill string decreases, and has its greatest effect when the drill string has substantially stopped. Sticking may also occur due to an element of the drill string locking with one of the elements surrounding the wellbore (i.e., stabilizer blades can stick in a discontinuity of the wellbore's wall).
The stick-and-slip effect is a low frequency process, as it typically involves more than one turn of the drill string. Its period may be from 10 sec to 0.5 seconds. The required torque to sustain the rotation is varying and must provide by the surface equipment such as top drive or rotary table. With long period effect, the torque required from the surface drive equipment may clearly vary. Transmission of this variable torque, however, modify twisting of the drill string along its length, as it is an elastic system. This effect of variable torque and twisting affects the stored potential energy in the drill string due to elastic deformation. As multiple inertias are present along the drill-string, the variation of rotational speed affects the rotational kinetic energy in the drill string. Such variation of potential and kinetic energies may be associated with torsional resonance along the drill string. Thus, over time, operation of the drill string can operate in pattern in which the drill string cyclically slows, with potential stops, and quickly speeds up. As a result of this pattern, the drill string experiences a series of spikes in speed and torque, and may reduce the life span of the drill string and the efficiency of the drilling operation. It is critical to mitigate these phenomenon's to limit potential damage on the components. The drilling industry has proposed various methods to limit the effect of “stick-and-slips”. However, it is also critical to detect and limit the effect of whirling and avoiding confusion with the stick-and-slip situation.